Sailboats usually incorporate a keel, which provides stability for the sailboat, and also prevents the sailboat from "sideslipping" in response to the force that is created by the wind.
Such a keel prevents "sideslipping" both by simply resisting lateral movement through the water, and also by providing a positive so-called "lifting" force. This is achieved by employing a keel having a foil section which is convex on both sides of the keel. Such a foil section operates somewhat in the manner of an aircraft wing, and responds to the forward movement of the sailboat through the water to provide a positive sideways force or lift, which in fact reacts against the sideslipping forces imposed by the wind. This effect is particularly noticeable when the sailboat is sailing with the wind ahead of the beam, i.e., when it is sailing so-called "up-wind."
Because a sailboat is sailed on either tack it is essential that the keel be capable of producing lift both on one side and the other and consequently it is given a convex shape or foil section on both sides.
It is however well known that the shaping of the keel with a foil section which is symetrically convex on both sides severely restricts the ability of the keel to provide lift on one side, without producing lift on the other side as well. In fact, it is fairly well established that the keel will produce lift only when the sailboat is moving both in a forward direction, i.e., along an axis running from the stem of the stern of the sailboat and is also moving slightly in a sideways direction as well so that the resultant actual track of the sailboat is at an angle of between 4.degree. and 8.degree. to its longitudinal axis. The precise reasons why this occurs are not entirely understood, but it is believed to be due to the fact that when moving in this slightly offset or diagonal manner, the water moves past the keel in a different manner on the respective windward and leeward sides of the keel, and that this difference produces a negative pressure on the windward side of the keel thereby producing lift, similar to the effect of an aircraft wing.
Clearly, however, the analogy between a keel which is convex on both sides, and an aircraft wing which is convex on the upper side and either concave or flattened on the under side is of only relatively limited assistance in explaining the function of the keel as the boat moves through the water.
It is however widely believed that if a keel could be made with a foil section like a wing, i.e., more or less flat on the leeward side of the boat, and convex on the windward side of the boat, then it would produce very much more lift, and would tend to cause the sailboat to follow a track which more closely approximated to the fore and aft axis of the boat, and did not produce so much sideslipping. However, there are a number of factors which in the past have prevented this. Clearly, such a result could be achieved by the use of two keels, one of which could be drawn upwardly into the boat while sailing in one direction and then lowered down again, while the other one was raised upwardly for sailing in the other direction. Indeed, there have in the past been proposals somewhat of this nature.
Another proposal has been the use of so-called "leeboards" or "bilgeboards." These are a form of boards, like a keel, which are in fact attached to the two sides of the boat, and are lowered down and raised up alternately as the boat moves from one tack to the other. Such leeboards have been proposed which are convex on one side and flat on the other. However, leeboards have not achieved any wide popularity in use, and introduced various other inefficiencies in hull design which more than offset the advantages gained by using improved foil sections.
The same design considerations apply to some extent to rudders for sailboats, which at present are usually made with a symetrically convex foil section on both sides, and also to movable keels known as centerboards.